The present invention relates to an empty/load type brake control system for a railway freight car and, more particularly, to an empty/load changeover valve device that employs a proportioning valve to load-adjust the brake cylinder pressure in the empty range of car weight.
Single capacity brake equipment produces a brake shoe force that is independent of car loading, thus making it difficult to achieve desirably higher braking ratios for a loaded car without exceeding an empty car braking ratio sufficient to cause wheel slide. Sliding wheels are undesirable from the standpoint of reduced braking retardation, and slid-flat wheels.
Special brake equipment is therefore necessary to increase the loaded car braking ratio without incurring the consequence of a wheel slide condition when braking an empty car. Such equipment automatically adjusts brake shoe force according to the load condition of the car. These special equipments fall into two primary categories, dual capacity empty/load braking and multiple capacity or continuously variable load braking.
In the dual capacity empty/load equipment, there are just two settings, one for "empty" braking and one for "load" braking, the changeover point between the "empty" and "load" settings being selected at some predetermined car weight, usually at 20% of the full load capacity weight. In arbitrarily selecting this changeover point, it will be appreciated that a given car can be generally under-braked by the reduced brake pressure when the car weight is in the upper end of the "empty" weight range, since essentially the same adhesion demand is available at the lower end of the "load" weight range at which maximum braking force is capable of being supported without sliding the car wheels.
In the variable load type equipment, braking pressure is proportioned according to the actual load, generally throughout the full range of car loading. It will be appreciated, however, that the proportioned brake pressure is selected in accordance with the maximum brake pressure (emergency) capable of being developed from the maximum running pressure normally carried by a train (110 psi.). Therefore, when making relatively light service brake applications or when making a maximum brake application from a relatively low running pressure (70 psi.), the proportioned brake pressure may be far less than that capable of being supported by the adhesion demand. Accordingly, less than optimum brake efficiency is realized under certain brake conditions with variable load type brake equipment, as well as single capacity equipment, in order to protect against wheel sliding on an empty car under maximum braking conditions.
In known dual capacity brake systems such as disclosed in U.S. Pat. No. 3,671,086, a proportioning valve arrangement is employed to reduce the braking pressure by a fixed ratio during empty car braking. In order to be compatible with the quick service limiting valve requirements for freight brake control valves, which assures that 8 to 12 psi. brake cylinder pressure will be developed from even the lightest service application, the aforementioned system withholds the empty/load sensing function until a predetermined minimum brake cylinder pressure has developed, generally 12 psi. However, since an equalizing volume is required in such proportional type dual capacity systems to maintain proper control valve operation, an inadvertent loss of braking pressure can occur when the empty/load changeover valve sensing function comes into play, due to the existing brake cylinder pressure being able to momentarily backflow into the equalizing volume. This occurs when a light service reduction is made, just sufficient to operate the load sensing valve on empty cars, in which case, the brake cylinder pressure can build up to 12 psi. and then be reduced back to the limiting valve pressure setting on each car, which can be as light as 8 psi. This is caused by the brake cylinder air flowing into the empty/load equalizing volume.